The invention relates to an alignment or positioning system for image-forming equipment used in manufacturing semiconductor devices and liquid crystal displays (LCD).
This background is provided to establish the context of the invention and is not to be interpreted as an admission that features and/or problems discussed herein were known or recognized by one of ordinary skill in the art at the time of the invention.
Many devices such as semiconductor circuits and liquid crystal displays are formed using lithographic equipment. It is necessary to repeatedly position a substrate beneath the optics of the lithographic equipment very precisely in order to accurately align each microscopic feature being formed in a new layer with other features previously formed individually in the multiple layers forming the semiconductor circuits or liquid crystal displays.
Complex alignment and positioning systems have been developed to precisely position the substrate beneath the lithographic optics. Conventional alignment systems can generally be classified into three categories: an off-axis alignment system for performing alignment without the mediacy of a projection unit, a throughthe-lens (xe2x80x9cTTLxe2x80x9d) alignment system designed to detect an alignment mark on a wafer through a projection unit without detecting any alignment mark on a reticle, and a through-the-reticle (xe2x80x9cTTRxe2x80x9d) alignment system for performing alignment through a projection unit that uses a mark on a reticle to align the wafer.
An off-axis alignment system is designed to detect an alignment mark on a wafer through an objective lens. The optics of the off-axis alignment system are separated from the projection unit by a predetermined distance.
A TTL alignment system is designed to detect an alignment mark on a wafer through the projection optics without detecting any alignment mark on a reticle. In lithographic equipment having a TTL or off-axis alignment system, the position of an alignment mark formed beforehand near each shot area on a photosensitive substrate in the previous step is detected by using both the off-axis or TTL alignment system, which detects no alignment mark on a reticle, and a stage position detection means such as a laser interferometer. In addition, a baseline amount which is the distance between the detection center of the alignment system and the optical axis of the projection unit is obtained in advance. The photosensitive substrate is sequentially aligned with the positions obtained by correcting the detected positions by the baseline amount, thereby effecting exposure on each shot area on the photosensitive substrate.
In lithographic equipment having a TTR alignment system, the position of a stage at which exposure is to be effected is not obtained, but alignment of a reticle and a photosensitive substrate is performed by simultaneously observing a mark on the photosensitive substrate and a mark on the reticle, thereby effecting exposure.
The above-mentioned TTR, TTL, and off-axis alignment systems are described in more detail in, e.g., U.S. Pat. Nos. 4,962,318 and 5,798,530, each of which is incorporated by reference in its entirety for all that it teaches.
The substrate, the substrate positioning stage, and the lithographic equipment each have features that are used to indicate the position of the substrate positioning stage in relation to the lithographic equipment. In one example of an off-axis alignment system, reference mirrors are placed on the substrate positioning stage and on the outside of the housing of the lithographic equipment by, for example, mounting them on INVAR(copyright) metal inserts in a brass ring plate attached to the bottom of the projection unit. Beams of laser light are reflected off the reference mirrors and are detected by sensors, and the position of the substrate positioning stage relative to the lithographic equipment is determined from the detected light. Motors on the substrate positioning stage move the stage until the wafer is properly positioned beneath the lithographic equipment.
There are many factors that inhibit the precise placement of the substrate, and thermal variations are one major source of inaccuracy in precise placement of the substrate. The light or charged particle beams produce heat as they are partially absorbed by the optics used to focus the beams onto the substrate. When a substrate is first positioned onto the substrate positioning stage, no energy has been absorbed by the optics since the beam is shut off. However, as the beam is rapidly turned on and off as the substrate is alternately exposed to the beam and repositioned by motors on the substrate positioning stage, the optics heat and cool, and the housing in which the optics are positioned expands and contracts. The lithographic equipment usually contains a sophisticated heat exchange system that maintains the temperature of the atmosphere surrounding the equipment essentially constant (within approximately xc2x10.05 degrees C. of set-point) and maintains the temperature of the projection unit itself essentially constant (within approximately 0.01 degrees C. of set-point), but the rate at which heat is added to the equipment changes sufficiently rapidly that the temperature of the housing in which the optics are contained fluctuates despite the temperatures of the atmosphere around the housing and the projection unit itself being held essentially constant.
Reference points that are mounted to the housing of the lithographic equipment move as the housing expands and contracts due to thermal expansion and contraction. Consequently, features external to the housing that are used as reference points for the axis of the lithographic equipment move relative to the axis. This movement of the reference points may introduce an inaccuracy into the positioning system, since the reference points do not maintain a fixed relationship to the axis of the lithographic equipment.
One system for compensating for movement of a reference feature is an off-axis objective, which determines the position of alignment markers on wafers and other substrates being processed to provide correct alignment of the substrate with the projection unit. The off-axis objective uses interferometers and mirrors attached to the objective to provide a signal representative of the amount of movement of the off-axis objective. Any movement is monitored periodically to detect and correct for changes in the location of the off-axis objective. A system that reduces or possibly eliminates this movement and the need to measure this movement would help to provide more accurate alignment of the substrate to the projection unit as well as a less-complicated and faster system.
There is consequently a need for equipment and methods that maintain a constant distance between the axis of the lithographic equipment and features that are external to the lithographic equipment and that act as reference points for the axis of the lithographic equipment. There is also a need for equipment that can provide better alignment of features formed using multiple layers and multiple lithographic steps.
It is one object of the invention to provide a reference assembly that mounts coaxially to a piece of equipment and provides a reference feature that is positioned a known and consistent distance from the axis of the piece of equipment, regardless of temperature fluctuations of the equipment.
It is another object of the invention to provide a method of positioning a target substrate beneath projection optics of lithographic equipment that utilizes a reference feature that is in an essentially fixed position to the equipment.
The invention in one embodiment provides a reference assembly that is mounted coaxially with an axis of a projection unit, wherein the reference assembly has a structure formed of a material having a low coefficient of thermal expansion, the structure has one or more reference features thereon, and the structure is mounted to a housing of the projection unit using expansion joints that compensate for the thermal expansion and contraction of the housing for the projection unit. A reference feature maintains at least a substantially fixed relationship to the axis of the projection unit as the system heats and cools during use. The invention also provides an exposure apparatus that incorporates the reference assembly with other equipment such as a substrate positioning stage and a projection unit.
The invention in another embodiment provides a method for positioning a target substrate beneath a projection optics system. The method utilizes a reference assembly of the invention to accurately establish the position of a substrate positioning stage carrying the target substrate after the position of the substrate is established by observing reference markers on the target substrate and after the position of the reference markers are correlated to the position of the substrate positioning stage at the time the reference markers are observed.
Among other factors, the invention is based on the technical finding that a coaxially-mounted reference assembly formed of a material having low coefficient of thermal expansion and attached to the housing of lithographic equipment by expansion joints that compress or expand as the equipment expands or contracts provides the ability to accurately place a substrate beneath the lithographic equipment despite changes in temperature that the equipment experiences. These technical findings and advantages and others are apparent from the discussion herein.